1. Technical Field
The invention relates generally to the field of wooden items, such as musical instrument components, gunstocks, tools, and the like, and is directed to a method of treatment of wood in preparation of manufacture of wooden items. More specifically, the invention is directed to a method for heat treating suitable wood to achieve desirable characteristics for the wooden items made thereof while preserving the aesthetics of natural wood.
2. Description of Prior Art
Wood used in the manufacture of musical instruments, gunstocks, tool handles, and other high end items needs to be both structurally sound and aesthetically pleasing. Especially for musical instruments and gunstocks, wooden components must be sufficiently hard, have low moisture content and low moisture absorption properties, must be highly stable (that is, resistant to shrinkage and swelling), and be insect resistant. The shrinking and expansion of components of a musical instrument can alter the sound of the instrument and prevent it from producing a musically pleasing sound, and the shrinking and expansion of a gunstock, however slight, can affect the accuracy of the firearm.
Because wood is often not sufficiently hard, with low moisture content and absorption properties, and may be unstable and susceptible to shrinking and expansion, manufacturers of high end items have recently opted to use synthetic materials, which are more likely to achieve the desired properties. Nevertheless, using wood for high end items is still desirable, for their aesthetics and tactile qualities, as well as historical fidelity, and therefore a method of manufacture of wooden items that overcomes the deficiencies of traditional wooden items is desired.
One method for decreasing the susceptibility of wooden items to moisture and rot is to chemically treat the wood before fashioning it into a finished item. A common method of chemically treating wood is the “pressure treatment” method, in which the wood is treated with chemicals such as arsenic and chromium (Chromate Copper Arsenate), alkaline copper quaternary (ACQ), or copper azole preservative, applied to the wood using a vacuum and pressure cycle to force the chemicals deep into the inner portions of the wood. Other chemicals may also be used. While this method tends to improve the weather resistance as well as insect and rot resistance of the wood, it does not address swelling and shrinkage issues. The toxicity of the chemicals used also renders this method less than desirable.
Another method for decreasing the susceptibility of wooden items to moisture and rot is to treat the wood in a non-pressurized manner with preservatives. These preservatives may be chemically based or derived from naturally occurring compounds, such as oils, and the preservatives are applied to the surface of the wood. While this method tends to be simpler than the pressure treatment method, and potentially uses less toxic preservatives, it fails to ensure a uniform application of the preservative into the inner portions of the wood. It also does not address swelling and shrinkage issues.
There is known in the art yet another method for decreasing the susceptibility of wooden items to moisture and rot, which is preferable to the above-described methods. Wood may be heat treated prior to being fashioned into a gunstock. European Patent Application EP 0 922 918 A1 (Aug. 3, 1998), to Lallukka, Tero, for “Method for heat treatment of timber”, discloses such a method for treating wood.
Wood is made up, generally, of cellulose, lignin, and extractives. Cellulose (and hemicelluloses) are carbohydrates that are structural components in wood. Cellulose constitutes 40-50% and hemicelluloses 25-35% of wood. The composition and contents of hemicelluloses vary from one wood species to another. During heat treatment, both groups undergo changes, but the majority of the changes occur in hemicelluloses. After heat treatment, the wood contains a substantially lower amount of hemicelluloses. As a result of this, the amount of fungi susceptible material is significantly lower, providing one reason for heat-treated woods improved resistance to fungal decay compared with normal kiln dried wood. With the degrading of the hemicelluloses, the concentration of water-absorbing components decreases and the dimensional stability of treated wood is also improved compared to normal kiln dried wood. The decomposition temperature of the hemicelluloses is about 200-260° C., and the corresponding temperature for cellulose is about 240-350° C. Lignin holds the wood cells together. Lignin constitutes 20-30% of wood. During heat treatment, bonds between components of lignin are partially broken. Of all wood's constituents, lignin has the best ability to withstand heat. Lignin's mass starts to decrease when the temperature exceeds 200° C. Wood also contains minor amounts of small-molecule constituents known as extractives. Extractives constitute less than 5% of wood. Extractives are not structural components in wood, and most of the compounds evaporate easily during the heat treatment.
Heat treating wood changes the structure of the wood in a manner which is desirable for the manufacture of many different kinds of high end wooden items. During heat treatment, wood undergoes mild pyrolysis, resulting in degradation of hemicelluloses and amorphous cellulose, modification of lignin structures, and evaporation of extractives from the wood. The lignin and hemicelluloses become less hygroscopic. Surface hardness increases, moisture is 10%-50% less than in untreated wood, resins dry out or evaporate, less absorption of moisture occurs, as well as reduced molding, improved weather resistance, and moisture deformation is reduced by 30% to 90% over untreated wood.
Thermally modified wood has a lower density than untreated wood. This is mainly due to the changes of the mass during the treatment when wood loses its weight. Density decreases as higher treatment temperatures are used. This leads to overall lighter weight of the wood. The strength of wood has a strong correlation with density. Because thermally modified wood has slightly lower density after the treatment, it is somewhat less strong than untreated wood. However, the change in the weight-to-strength ratio is minimal. The strength of wood is also highly dependent on the moisture content and its relative level below the grain saturation point. Thermally modified wood benefits due to its lower equilibrium moisture content. Heat treated wood is therefore sufficiently strong for use in high end wooden items.
Heat treatment also significantly reduces the tangential and radial swelling of wood. Heat-treated wood consequently has very low shrinkage. The water permeability of heat-treated wood is 20-30 percent lower than that of normal kiln dried wood. Thermally modified wood is resistant to insects (which are attracted to the extractives of untreated wood; such extractives are largely evaporated away during heat treatment).
Species of tree which are suitable for thermal modification include American Beech (Fagus Grandifolia), Red Maple (Acer Rubrum), Black Walnut (Juglans Nigra), Hard maple (Acer Saccharum), Turkish Walnut a/k/a English Walnut (Juglans Regia), California Walnut (Juglans Californica), Yellow Birch (Betula Alleghaniensis), and Claro Walnut (Juglans Hindsii). Other species are also suitable for thermal modification.
Some woods naturally exhibit the preferred characteristics described above. For example, with regard to wood hardness, Madagascar ebony (Diospyros celebica) measures 3220 on the Janka Hardness Scale. (The Janka Hardness Scale measures the resistance of a type of wood to withstand denting and wear, measuring the force required to embed an 11.28 mm (0.444 in) steel ball into wood to half the ball's diameter; the measurement is expressed in pounds-force (lbf).) In contrast, the hardness of hard maple (A. saccharum) is only 1450 on the Janka Hardness Scale. A soft wood such as eastern white pine (Pinus strobus) has a hardness of only 380 on the Janka Hardness Scale.
However, ebony falls under the Lacey Act of 1900, which was amended in 2008 to include provisions to curtail illegal logging. The amended Lacey Act prohibits all trade in plant and plant products (e.g., furniture, paper, or lumber) that are illegally sourced from any U.S. state or any foreign country; requires importers to declare the country of origin of harvest and species name of all plants contained in their products; and establishes penalties for violation of the Act, including forfeiture of goods and vessels, fines, and jail time. Because ebony is relatively rare, procurement of ebony often is not done consistent with the provisions of the Lacey Act. Ebony is thus difficult to obtain, and the legal supply is not sufficient to meet demand.
As a result, manufacturers have a need for a substitute wood that meets the characteristics of ebony, but which is in greater supply. One species that is an acceptable substitute is ipe (Tabebuia Serratifolia), a common specie of wood found abundantly in South America. Ipe has a hardness on the Janka Hardness Scale of 3684, making it sufficiently hard. However, while ipe has a naturally dark color it is nowhere near the true black of ebony. Ipe therefore must be modified to achieve proper coloration. Thermally modifying ipe brings it closer to the color of ebony than any other species of wood while retaining the hardness that is necessary. In addition, thermal modification dries the wood and reduces its susceptibility to shrinkage or swelling, as well as making it more insect resistant. In addition to ipe, there are a few other species of wood that are sufficiently hard and sufficiently abundant that they can be acceptable substitutes for ebony when they are thermally treated. These include purpleheart (Peltogyne paniculata), Brazilian walnut (Swartzia tomentosa), and cumaru (Dipyeryx odorata).
In summary, heat treating wood reduces its moisture content; it reduces the ability of the wood to absorb environmental moisture; it increases the surface hardness of the wood; it increases the overall stability of the wood (that is, minimizes expansion and shrinkage); it causes the wood to become less dense, and therefore lighter; and it makes the wood less susceptible to rot and insect predation. It also allows for aesthetically pleasing coloration changes to the wood. Heat treatment of wood further accomplishes these desirable characteristics without the use of toxic chemicals.
From the foregoing it is evident that there is a need for a method of treatment of wood for the manufacture for wooden items, particularly components for musical instruments, gunstocks, tool handles, and other high end items.
It is therefore an objective of the present invention to provide a method of heat treatment of wood for the manufacture of wooden items.
It is a further objective of the present invention to provide a method of heat treatment of wood for the manufacture of components for musical instruments, gunstocks, tool handles, and other high end items.
It is a further objective of the present invention to provide a method of heat treatment which darkens the color of wood.
It is a further objective of the present invention to provide a method that increases the surface hardness of the wood.
It is a further objective of the present invention to provide a method that reduces the moisture content of wooden items to minimize expansion and shrinkage and to increase the stability thereof.
It is a further objective of the present invention to provide a method that makes the wood less susceptible to environmental moisture.
It is a further objective of the present invention to provide a method that makes the wood less susceptible to rot and insect predation.
It is a further objective of the present invention to provide a method that decreases the density and therefore the weight of the wood.
It is a further objective of the present invention to provide a method which does not use toxic chemicals to treat the wood.
Other objectives of the present invention will be readily apparent from the description that follows.